Papermaking is the process of converting a raw cellulosic material, typically wood pulp, into paper or board. Hereafter, the terms paper product, pulp product, or simply product will be used to denote any final product made from a cellulosic material. In other words, a paper or pulp product can be either paper or board. Paper products can be made from a variety of raw materials, the most popular of which are wood-containing materials. Paper products may also be produced from non-wood materials such as straw, cotton, etc . . .
Many methods are available for preparing a virgin pulp slurry from wood and non-wood materials. Such methods are generally classed as either mechanical, chemical, or hybrid. In addition to virgin pulps, recycled pulps, also called secondary fibers, are available for making a pulp slurry. Examples of sources of secondary fibers are old newspapers, old corrugated containers and mixed office waste. Paper products can be made from 100% recycled material, 100% virgin material, or mixtures of both virgin and recycled material.
Hereafter, terms known in the papermaking art will be used and definitions of those terms can be found in the Appendix hereto. Paper products are made by taking a pulp slurry, a mixture of water and cellulosic material, and running it through a series of process steps, among which are refining, mixing, pumping, cleaning, diluting, thickening, draining, pressing, drying, and winding. During processing, various additives are incorporated into the furnish; examples of these additives are dyes, fillers, starches, pH adjusters, and sizes.
Two papermaking processes are generally in use: an acid-based process and an alkaline-based process. Alkaline papermaking involves the addition of neutral or alkaline chemicals, typically calcium carbonate and alkylketene dimer (AKD) size, to the pulp furnish. By contrast, acid papermaking is a process in which the pulp furnish pH is slightly acidic. Papermakers generally use aluminum sulfate and rosin size in acid-based papermaking.
Aluminum sulfate, or papermaker's alum, is a common filler in acid papermaking and serves two main functions. It reduces pH, which improves drainage on the wire section of the paper machine, and it fixes additives such as dyes and rosin, thus improving retention of fines, sizes, and other fillers. Alum, or more specifically, the hydrated complex of the aluminum ion, undergoes hydrolysis in solution according to the following reaction: EQU [Al(H.sub.2 O).sub.6 ].sup.3+.fwdarw.[Al(H.sub.2 O).sub.5 (OH)].sup.2+ +H.sup.+ (1)
This reaction represents an acid dissociation and is the method by which the aluminum sulfate adjusts pH in the acid papermaking process. pH adjustment is necessary for optimal retention of the size. In a typical alum-rosin sizing system, pH is lowered below 5.0, usually around 4.5. A consequences of this pH adjustment is that drainage on the wire section of the paper machine is improved.
The aluminum product of the above reaction, [Al(H.sub.2 O).sub.5 (OH)].sup.2+, is crucial in the next step of rosin sizing. The product undergoes a series of complicated polymer reactions to serve as a mordant which combines with the rosin to form an insoluble complex. This complex in turn fixes the rosin to the fiber structure and enhances retention of the rosin. The complex is also useful in retaining fines because the complex can bridge multiple cellulose molecules, forming a larger compound which is retained on the wire rather than washed out of the solution.
Because of the dual nature of papermaker's alum and the ease with which it is administered, papermakers tend to overfeed alum to accomplish retention of the size, adjustment of pH, and an increase in drainage. This propensity is especially true in the manufacture of paper products from wastepaper composed primarily of acid-sized paper products. Wastepaper of this composition, typically kraft papers such as cardboard and linerboard, already contains sufficient alum to complex with a portion of the additional size that will be added in stock preparation. Therefore, additional alum serves primarily to adjust the pH of the stock solution and to increase wire drainage.
Overuse of aluminum sulfate, however, can present a number of problems, namely:
aluminum sulfate is a relatively hazardous chemical, increasing the potential for possible operator injury; PA1 it is corrosive and increases the maintenance cost of papermaking equipment; PA1 it is relatively expensive; PA1 it reacts to form sulfuric acid, making it easy to over-acidify the pulp solution; PA1 excess aluminum sulfate leaving with the finished product reacts with moisture to produce sulfuric acid which attacks the pulp fibers and degrades the paper product; PA1 the sulfate portion of the aluminum sulfate can accumulate in the white water system, causing production and maintenance problems; and PA1 excess aluminum sulfate can form flocs in the fiber suspension and promote defects in sheet formation and in the finished paper product.
U.S. Pat. No. 1,753,690 to Brown is indicative of the prior art which employs aluminum sulfate to assist in the production of paper. The Brown patent discloses the use of a mixture of waste wax paper and fresh fibers, such as mixed paper or sulfite paper fiber. The mixture is heated to a temperature sufficiently high to soften the rosin contained in the wastepaper. The heated mixture of fibers is beaten to effect a disintegration of the paper, additional rosin is added and a solution of sodium silicate is also added to produce a dispersion. The mixture is then cooled to a temperature before the gelation point of the dispersed particles. Aluminum sulfate is then added in an amount to produce a slightly acid reaction. The aluminum sulfate reacts with the sodium silicate to produce a flocculent precipitate which carries the dispersed particles of waterproofing material that are contained in the wastepaper into the fibers.
U.S. Pat. No. 5,505,819 to DeWitt discloses a method of using acid, preferably phosphoric acid, in a papermaking process, in conjunction with bentonite and a suitable polymer. Dewitt shows that increased drainage can be attained by controlling the pH of an alkaline or neutral papermaking process in the range of 6.7 to 7.5.
U.S. Pat. No. 5,378,322 entitled "Carbon Dioxide in Neutral and Alkaline Sizing Processes" to Hornsey discloses a method for sizing paper with alkylketene dimer size and CO.sub.2 in an alkaline environment. Hornsey nowhere indicates that CO.sub.2 can be used with acid sizes such as alum.
As above indicated, excessive use of aluminum sulfate not only degrades the paper's quality, but also increases the maintenance costs of the papermaking equipment. Further, if the pH begins to fluctuate as a result of the strong acidic characteristic of the aluminum sulfate product, i.e., sulfuric acid, such fluctuations directly affect de-watering and impair the papermaker's ability to control the process.
Accordingly, it is an object of this invention to provide an improved papermaking process which minimizes the addition of aluminum sulfate to the pulp furnish.
It is another object of this invention to provide an improved method of papermaking which improves the paper product's ultimate resistance to deterioration.
It is yet another object of this invention to provide an improved method of papermaking which reduces the potential for defects resulting from flawed dispersion in the finished product.